[Out of Stock] One of the rarer aircraft that the US Air Force (USAF) operates is the E-4B Advanced Airborne Command Post, with the first example entering service in 1974. There are only four of these Boeing 747 aircraft in existence, and they remain on hand to act as survivable airborne command posts for the US President and Secretary of Defence. The E-4B is based on a Boeing 747-200 airframe, and it can stay aloft via an aerial refuelling system. All four E-4Bs are operated by the 1st Airborne Command and Control Squadron (1ACCS) of the 55th Wing at Offutt Air Force base in Omaha, Nebraska. These aircraft are hardened against electromagnetic pulses (EMP) caused by nuclear blasts, plus they have nuclear- and thermal-effects shielding. Valued at USD$250 million each, the E-4B fleet began undergoing an upgrade in 2005. A mission requires at least 48 crewmen, and one aircraft is on alert 24 hours a day ready for a national emergency. The E-4B's unique electronic and communications capabilities ensure it will remain in service until at least 2015, despite the threat of nuclear war receding.